Seat belt systems were developed to secure the occupant of a vehicle against harmful movement that may result during a collision or a sudden stop. The systems reduce the likelihood and severity of injury in a traffic collision by stopping the vehicle occupant from hitting hard against interior elements of the vehicle or other passengers (the so-called second impact), by keeping occupants positioned correctly for maximum benefit from the airbag and by preventing occupants being ejected from the seat of the vehicle.
A traditional seat belt system comprises a seat belt for restraining the occupant in the seat, a tongue plate arranged on the seat belt, a belt buckle detachably engagable with the tongue plate, and a belt winder for tightening or loosing the seat belt. The tongue plate is slidably fixed to the seat belt, with a stopper fixed to the seat belt to stop the tongue plate from sliding along the seat belt all the way to a lower anchorage. The stopper keeps the tongue plate at a rest position on the seat belt when the seat belt is not in use and is fully retracted by the belt winder without slack. When the occupant is about to wear the seat belt, he must take the tongue plate or seat belt and then apply a certain force in order to pull out the seat belt from the belt winder. However, as the seat belt and the tongue plate slidably mounted thereon is located laterally and rearwardly of the occupant, the occupant has to turn around to reach the tongue plate or seat belt and use an unnatural posture to apply the force. It is uncomfortable and inconvenient for the occupant. To solve the problem, a seat belt presenter for moving the tongue plate to a more convenient position was proposed.
The seat belt presenter uses an electric motor as a source of power. FIG. 4 illustrates a winding scheme of a traditional permanent magnet direct current (PMDC) motor used for driving the seat belt presenter. The rotor windings comprise a plurality of coils wound about teeth of the rotor core and electrically connected to segments of a commutator. For example, coil C1′ is wound about teeth T1 and T2 and connected to segments S1 and S2. Coil C2′ is wound about teeth T2 and T3 and connected to segments S2 and S3. The PMDC motor comprises two brushes to feed electrical current to the rotor windings. The rotor windings form two parallel branches. When a coil such as coil C1′ is open, one of the branches will be open. The motor as well as the seat belt presenter will malfunction.
Therefore, there is a desire for a seat belt presenter with improved reliability.